It is known to prepare hot-melt adhesive compositions from polystyrene-polyisoprene-polystyrene and polystyrene-polybutadiene-polystyrene block copolymers. Linear polystyrene-polyisoprene-polystyrene block copolymers, hot melt adhesive compositions based on these block copolymers, articles produced from these adhesives, are disclosed in U.S. patent application Ser. No. 393,545, supra. The linear polystyrene-polyisoprene-polystyrene block copolymer, or linear, pS-pI-pS block copolymer, disclosed by this application is characterized by any of the formulas: (1) pI-(pS-pI).sub.n, where n is 2, or greater than 2; (2) pS-(pI-pS).sub.n, where n is 1, or greater than 1; or (3) (pS-pI).sub.n, where n is 2, or greater than 2; wherein, is any of formulas (1), (2) or (3), pS is a polystyrene block having an average molecular weight ranging from about 12,000 to about 20,000, preferably from about 14,000 to about 19,000, pI is a polyisoprene block having an average molecular weight ranging from about 30,000 to about 70,000, preferably from about 35,000 to about 60,000, the overall molecular weight of the block copolymer ranges from about 60,000 to about 110,000, preferably from about 70,000 to about 95,000, and the polystyrene block pS components are present in an amount of at least about 27 parts to about 50 parts, preferably from about 35 parts to about 45 parts, per 100 parts by weight of the block copolymer. The pS-pI-pS block copolymer, in all embodiments is characterized by the presence of a pI block, or polyisoprene block, located between two pS blocks, or polystyrene blocks which may or may not be terminal endblocks. The pS-pI-pS block copolymer can thus be either a triblock or multi-block copolymer, though the triblock copolymer is preferred.
The hot melt adhesive composition is constituted of the pS-pI-pS block copolymer, a compatible primary tackifier resin, preferably also a secondary tackifier or plasticizing resin or plasticizing oil, and stabilizer. These block copolymers, when blended in the requisite proportions with these components, produce adhesives with high shear holding power and shear adhesion failure temperature, and a low overall molecular weight sufficient to provide low viscosity. These adhesives have, inter alia, been found admirably suitable for the construction of disposable articles wherein the adhesive is applied as a continuous or discontinuous phase between members, e.g., via brushing, spraying or air-extrusion, between members to be bound together, particularly disposable articles of multi-line construction wherein the adhesive is supplied as fine parallel longitudinal strips, or as a multi-dot pattern of adhesive droplets, to bond together a moisture impervious outer polyethylene or polypropylene sheet and an inner moisture adsorbent sheet, or tissue, as used in diaper constructions. They have also been found suitable for use in the construction of sanitary napkins, bed pads, and with or without the addition of other materials, are useful for packaging and carton sealing, magazine and book lining, or book binding, or as elastic glues generally.
Adhesives formed from this type of pS-pI-pS block copolymer has been found admirably suitable, e.g., in the production of "multi-line" (or multi-dot) constructions. Hot-melt adhesives applied in the form of fine parallel longitudinal strips (or as patterns of dots) have been found to possess sufficient adhesive and cohesive strength to provide high bond strength values so that when subjected to stress the constructions cannot be easily separated. Moreover, the adhesives can withstand high mixing and application temperatures without thermal degradation and loss of adhesive properties, and have good heat and oxidation resistance on aging. The adhesives also perform well at moderate temperatures, which is required since the disposable articles are worn at body temperature; and they also perform well at higher temperatures, which is also required since the constructed articles must also be exposed to elevated temperatures during warehousing and shipping. Furthermore, these hot-melt adhesives have low adhesive viscosity such that they can be applied at low temperature in order to avoid distortion of the polyethylene or polypropylene substrates to which the adhesive is applied. Nonetheless, there remains a need for hot-melt adhesives useful in magazine and book binding, elastic gluing operations generally, hot-melt adhesive compositions useful in the assembly of multi-line constructions, and disposable articles of multi-line construction formed from improved hot-melt adhesive compositions.